allow visiting other people's display/ links if I have a copy of the post

Rework mod/display a bit. Primarily, in the case where one person visits another person's display/1234 link, the script will now check if the visitor has his own local copy before giving a "Permission denied." If there is a local copy, that will be displayed.

Also, support for linking using a URI (display/username/uri) was already there, so I removed an intval() that prevented it from being used.

I also tried to beef up the html encoding for the meta tags a bit more.